LONDON (AP) — British voters cast ballots Thursday in local and regional elections that could shake up the country's politics and deliver a heavy blow to embattled Prime Minister Keir Starmer.

Starmer's center-left Labour Party is bracing for big losses in polls that will choose about 5,000 local councilors and a handful of mayors across England, as well as semiautonomous parliaments in Scotland and Wales. They are the biggest set of elections since Labour swept to power in a landslide in July 2024, and Starmer's opponents have painted them as a midterm referendum on the prime minister.

READ MORE: Local elections could hasten exit of embattled British Prime Minister Starmer

Polls will close at 10 p.m. (2100 GMT), and some local authorities will count ballots overnight, but the bulk of the results are likely to be declared on Friday afternoon.

A rout could trigger moves by restive Labour lawmakers to oust a leader who led them to power less than two years ago. Even if Starmer survives for now, many analysts doubt he will lead the party into the next national election, which must be held by 2029.

Starmer's popularity has plunged after repeated missteps since he became prime minister in July 2024. His government has struggled to deliver promised economic growth, repair tattered public services and ease the cost of living — tasks made harder by the U.S.-Israeli war with Iran, which has choked off oil shipments through the Strait of Hormuz.

The prime minister has been further hurt by his disastrous decision to appoint Peter Mandelson, a scandal-tarnished friend of Jeffrey Epstein, as Britain's ambassador to Washington.

READ MORE: Starmer avoids ethics probe over Mandelson appointment but challenges lie ahead

Labour is defending about 2,500 seats on English local councils, and party members are apprehensive it may lose many of them.

Starmer already survived a crisis in February, when some Labour lawmakers, including the party's leader in Scotland, urged him to quit over the Mandelson appointment.

He has vowed to serve out his five-year term, but a bad result could spark a challenge from a high-profile rival such as Health Secretary Wes Streeting, former Deputy Prime Minister Angela Rayner or Greater Manchester Mayor Andy Burnham. Alternately, Starmer could face pressure from the party to set a timetable for his departure after an orderly leadership contest.

Hard-right Reform UK expected to win big

Luke Tryl of pollster More in Common said the local elections are likely to see "the total collapse of the traditional two-party system" that was dominated for decades by the Labour and Conservative parties.

The big winner is expected to be hard-right party Reform UK, led by Nigel Farage, which is aiming for working-class, former Labour strongholds in England's north and on London's outer edges with its anti-establishment, anti-immigration message. The Green Party is also likely to gain hundreds of council seats in urban centers and university towns.

The main opposition Conservative Party is also expected to lose ground, with the centrist Liberal Democrats making some gains.

Starmer didn't even mention the Conservatives in his final preelection message, framing it as a choice between "progress and a better future" under Labour and "the anger and division offered up by Reform or empty promises from the Greens."

Farage said on the eve of the election that a strong result for Reform would mean Starmer is "gone by the middle of summer."

Both Reform UK and the Greens have grown rapidly in the last year or two, and are facing increased scrutiny as a result. Farage is facing questions over a 5 million pound ($6.8 million) donation from a cryptocurrency billionaire that he accepted in 2024, but did not declare. He says it was a personal gift.

The environmentalist Greens, who have stressed their pro-Palestinian credentials under self-described "eco populist" leader Zack Polanski, have fired several candidates for antisemitic social media posts.

Reform also is eyeing breakthroughs in Scotland and Wales, though pro-independence nationalists the Scottish National Party and Plaid Cymru are likely to form governments in Edinburgh and Cardiff.

"Labour's going to lose to Reform in some places, Greens in others, and here and there they'll lose one or two seats to the Liberal Democrats and the Conservatives as well," said Tony Travers, a professor of government at the London School of Economics. "They're fighting on four fronts in England — five in Wales and Scotland."